4D DSA imaging is a new promising imaging method which can be used to represent the spatiotemporal diffusion of the blood whilst using contrast agent inside a 3D vascular tree.
The 4D DSA imaging method has been described for example by Prof. Dr. Charles A. Mistretta et al. in “4D-DSA and 4D Fluoroscopy: Preliminary Implementation”, published in Proceedings of the SPIE, 2010, Vol. 7622, pp. 762227-1 to 8. In the case of the 4D DSA approach, the dynamic behavior of the contrast agent in the vessels is extracted from the rotational angiography projections and said behavior is superimposed by means of perspective back projection in a static 3D image of the vascular tree. In this situation, a time-resolved series of 3D data sets is produced; associated with each point in time in this series is a volume data set which represents the filling of the vascular tree with contrast agent at the respective point in time.
Such 4D DSA imaging methods are used in order to visualize the blood flow in a vessel section or vessel segment of a blood vessel, which contains a pathological, in other words an abnormal change. Such a pathological change in the vessel section is present for example in the form of an aneurysm, in other words an abnormal, locally limited, often voluminous extension. An aneurysm can occur in particular in a blood vessel in the area of the brain or the heart; the occurrence of an aneurysm is however in general not restricted to a specific body region. The clinical significance of an aneurysm which for example is localized in the brain consists in particular in the danger of a rupture, in other words a tearing or splitting, which can for example result in bleeding and thromboses. In medicine today the dynamics of the blood flow in an aneurysm are frequently considered to be an important factor for the pathogenesis of the aneurysm, in other words for its origin and development.
An angiography system for carrying out such a 4D DSA imaging method is known for example from U.S. Pat. No. 7,500,784 B2, which is described with reference to FIG. 1.
FIG. 1 shows a monoplanar X-ray system illustrated as an example having a C-arm 2 held by a stand 1 in the form of a six-axle industrial or jointed arm robot, at the ends of which C-arm 2 are mounted an X-ray source, for example an X-ray emitter 3 having an X-ray tube and collimator, and an X-ray image detector 4 as the image recording unit.
By means of the jointed arm robot known for example from U.S. Pat. No. 7,500,784 B2 which preferably has six axes of rotation and thus six degrees of freedom, it is possible to spatially adjust the C-arm 2 as desired, for example by rotating it around a rotation center between the X-ray emitter 3 and the X-ray detector 4. The angiographic X-ray system 1 to 4 according to the invention can be rotated in particular around rotation centers and axes of rotation in the C-arm plane of the X-ray image detector 4, preferably around the center point of the X-ray image detector 4 and around axes of rotation intersecting the center point of the X-ray image detector 4.
The known jointed arm robot has a base frame which is fixedly mounted for example on a floor. Attached thereon is a carousel capable of rotation around a first axis of rotation. On the carousel is attached a robot rocker capable of swiveling around a second axis of rotation, to which is affixed a robot arm capable of rotation around a third axis of rotation. At the end of the robot arm is attached a robot hand capable of rotation around a fourth axis of rotation. The robot hand has a retaining element for the C-arm 2, which is capable of swiveling around a fifth axis of rotation and is capable of rotation around a sixth axis of rotation running perpendicular to the latter.
The implementation of the X-ray diagnostic device is not dependent on the industrial robot. Normal C-arm units can also be used.
The X-ray detector 4 can be a rectangular or square, flat semiconductor detector which is preferably produced from amorphous silicon (a-Si). Integrating and possibly counting CMOS detectors can however also be used.
A patient 6 to be examined is situated in the beam path of the X-ray emitter 3 on a table surface 5 of a patient support table as the object under examination. Connected to the X-ray diagnostic device is a system control unit 7 having an imaging system 8 which receives and processes the image signals from the X-ray image detector 4 (control elements are for example not illustrated). The X-ray images can then be viewed on displays of a monitor rack 9. A processing unit 10, the function whereof will be described in more detail, is furthermore provided in the system control unit 7.
Instead of the X-ray system illustrated for example in FIG. 1 having the stand 1 in the form of the six-axle industrial or jointed arm robot, the angiographic X-ray system can also have a normal ceiling- or floor-mounted fixture for the C-arm 2.
Instead of the C-arm 2 illustrated by way of example, the angiographic X-ray system can also have separate ceiling- and/or floor-mounted fixtures for the X-ray emitter 3 and the X-ray detector 4 which for example are rigidly coupled electronically.
In order to acquire 4D angiography sequences, a rotational angiogram is first produced using the monoplanar X-ray system illustrated in FIG. 1 and from this is reconstructed the 3D vascular tree of the vessels filled with contrast agent. In order to capture the time components, 2D DSA sequences of the vessels filled with contrast agent are produced and back projected into the reconstructed 3D vascular tree.
For a better understanding of the production of the 2D DSA sequences, a time-intensity curve or contrast-intensity curve for a vessel section with characteristic values drawn in is illustrated by way of example in FIG. 2, wherein the blood flow is plotted as intensity I over the time t. After a noise-type progression of the bolus curve 11 of the contrast agent, the intensity I increases up to the intensity maximum 12 (Imax) in order to then drop away again to a mean noise level 13. The bolus curve 11 is furthermore characterized by its full width at half maximum (FWHM) 14 which lies between the mean rise and the mean fall of the bolus curve 11.
The arrival time 15 (Trise) is the time which elapses up to the occurrence of the contrast agent bolus at the examined location and thus up to the rise of the bolus curve 11. The mean rise time 16 (Trise,FWHM) is the time which elapses up to the occurrence of the full width at half maximum 14 of the bolus curve 11, in other words until the bolus curve 11 has reached the half of the intensity maximum 12 (Imax). The time up to the intensity maximum 12 (Imax) is referred to as maximum time 17 (tmax, time to maximum). The rise time 18 or wash-in time (twash in) denotes the steep rise of the bolus curve 11. The fall of the bolus curve 11 is denoted by the fall time 19 or wash-out time (twash out). The duration of the contrast agent bolus is denoted by the bolus or maximum time 20 (tpeak).
In the case of the 4D DSA imaging method the 3D vascular tree is therefore firstly reconstructed from a rotational angiogram of the vessels filled with contrast agent and reproduced completely intensified or enhanced in the stationary state of opacity. The dynamic propagation of the contrast agent in the 3D vascular tree is then captured from a series of 2D DSA recordings in a wash-in or wash-out phase, which is back projected into the reconstructed 3D volume.
In this situation, the acquisition of the 4D angiography sequences can take place from projections of a rotational angiogram on the basis of superimpositions by means of perspective back projection in a static 3D image of the vascular tree. In this situation, a time-resolved series of 3D data sets is produced; associated with each point in time in said series is a volume data set which represents the filling of the vascular tree with contrast agent at the respective point in time.